Build the llvm support libraries (libcxx, libcxxabi) from scratch
without using the existing llvm libraries. This is the same spirit and
similar implementation as the "useLLVM" bootstrap in llvm package
sets. Critically it avoids having libcxxabi provided by the cc-wrapper
when building libcxx, which otherwise results in two libcxxabi
instances.
$ otool -L /nix/store/vd4vvgs9xngqbjzpg3qc41wl6jh42s9i-libc++-7.1.0/lib/libc++.dylib
/nix/store/vd4vvgs9xngqbjzpg3qc41wl6jh42s9i-libc++-7.1.0/lib/libc++.dylib:
/nix/store/vd4vvgs9xngqbjzpg3qc41wl6jh42s9i-libc++-7.1.0/lib/libc++.1.0.dylib (compatibility version 1.0.0, current version 1.0.0)
/nix/store/gmpwk5fyp3iasppqrrdpswxvid6kcp8r-libc++abi-7.1.0/lib/libc++abi.dylib (compatibility version 1.0.0, current version 1.0.0)
/nix/store/3hn7azynqgp2pm5gpdg45gpq0ia72skg-libc++abi-7.1.0/lib/libc++abi.dylib (compatibility version 1.0.0, current version 1.0.0)
/nix/store/1nq94scbxs6bk7pimqhvz76q6cfmbv97-Libsystem-osx-10.12.6/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1226.10.1)
Additionally move some utilities (clang, binutils, coreutils, gnugrep)
to the stage layers so they can be replaced before the final
stdenv. This should cause most of stage4 to be built from the
toolchain assembled as of stage3 instead of the bootstrap toolchain.
fece3eb2e9